Semiconductor structure with metal migration semiconductor barrier layers and method of forming the same

ABSTRACT

A semiconductor structure includes a semiconductor substrate, a semiconductor active region, a semiconductor contact layer, at least one metal migration semiconductor barrier layer, and a metal contact. The metal migration semiconductor barrier layer may be embedded within the semiconductor contact layer. Furthermore, the metal migration semiconductor barrier layer may be located underneath or above and in intimate contact with the semiconductor contact layer. The metal migration semiconductor barrier layer and the semiconductor contact layer form a contact structure that prevents metals from migrating from the metal contact into the semiconductor active layer during long-term exposure to high temperatures. By providing a robust contact structure that may be used in semiconductor structures, for example in solar cells that power spacecraft or terrestrial solar cells used under concentrated sunlight, the high temperature reliability of the semiconductor structure will be improved and the operation time will be prolonged.

BACKGROUND OF THE INVENTION

The present invention generally relates to semiconductor structures and,more particularly, to semiconductor structures including metal migrationbarrier layers preventing metal migration from a contact layer into anactive layer of a semiconductor structure and method of forming thesame.

While manufacturing semiconductor devices, such as III-V semiconductorbased multifunction solar cells, electrical contacts are formed onindividual semiconductor devices and connected together to perform thedesired circuit functions. The electrical contact formation andconnection process involves metal layers and is generally called“metallization”. Metal contact layers containing multiple thin layerswith alternating composition of, for example Ti/Au/Ag, areconventionally used for providing electrical contact to semiconductordevices.

FIG. 1 provides a schematic cross sectional view of a typical prior artsemiconductor structure 10. The semiconductor layers may be depositedduring a process typically referred to as semiconductor wafer growthprocess. In this wafer growth process, semiconductor active layers 11are generally deposited onto a semiconductor substrate 12. Asemiconductor contact layer 13 may be then deposited onto thesemiconductor active layers 11. Typically, the semiconductor contactlayer 13 is the last layer to be deposited during the wafer growthprocess. After completion of the growth process, a metal contact 14 isgenerally deposited in a separate process typically referred to asdevice fabrication process.

Metal films used for electrical contacts on semiconductor devices maymigrate into the semiconductor active region under certain environmentalconditions, such as long time exposure to high temperatures, causingproduct reliability concerns. FIG. 2 shows a schematic cross sectionalview of a typical prior art semiconductor structure 10 wherein metalprotrusion 15 from the metal contact 14 into the semiconductor activelayers 11 has occurred. The metal protrusion 15 may cause thesemiconductor structure 10 to fail. Consequently, a semiconductorstructure that blocks or prevents metal migration from the metal contactinto the semiconductor active layer is highly desirable and is importantfor the operation life, and high temperature reliability of suchsemiconductor structure 10, for example, a solar cell.

The most common method to mitigate the existing problem is to use ametal barrier layer as part of the entire metal contact structure, suchthat the barrier metal is placed underneath the main metal layer 14. Inprior art, platinum (Pt), palladium (Pd), and similar elements orcompounds that have high temperature stability are commonly used asmetal barrier layers. The main purpose of the metal barrier layer is toprevent diffusion of conductive material from the metal contact 14, suchas Au or Ag, for example, into the semiconductor active layers 11.

However, this prior art approach may still be susceptible to metaldiffusion under certain environmental conditions, such as exposure ofthe semiconductor structure to high temperatures for prolonged periodsof time. Since the semiconductor contact layer 13 generally is composedof a single homogeneous structure, the semiconductor contact layer 13may not provide any obstacles to the metal protrusion 15 once the metaldiffusion process has started. Therefore, once metal migration from themetal contact 14 into the semiconductor contact layer 13 has started, itis likely that the metal will migrate through the entire semiconductorcontact layer 13 and reach the active semiconductor region.Consequently, it will be only a matter of time until the metal reachesthe semiconductor active layers 11 and causes the structure to fail.Therefore, it is necessary to find appropriate materials that can beincorporated within the contact structure of a semiconductor structureand that enable suppression of the metal migration into the activeregion of the structure even during long time exposure to hightemperatures, as found for example during space and terrestrialapplications.

Prior art multijunction solar cells 11 (as shown in FIG. 3) providepower to many satellites and other spacecraft. FIG. 3 shows aperspective view of a cell-interconnect-coverglass assembly 30 of atypical prior art multijunction solar cell 31. A plurality ofinterconnects 32 may be provided at one edge of the solar cell 31. Theinterconnects 32 may be welded on top of the metal contact 14, as shownin FIGS. 1 and 2. A coverglass 33 may be installed to protect the solarcell 31 and the interconnects 32 from radiation in space. Presently,metal protrusion 15 from the metal contact 14 through the semiconductorcontact layer 13 and into the semiconductor active layers 11, asillustrated in FIG. 2, may be observed more readily in the areas wherethe interconnects 32 are connected with the solar cell 31. For spaceapplications, such as the operation of III-V based multijunction solarcells mounted on spacecraft, the device operation life under extremeenvironmental conditions, such as exposure to relatively hightemperatures for relatively long times, is of very high importance.Therefore, a prolonged life, a higher performance, an improved hightemperature reliability, and stability of the solar cells 31 wouldresult in a prolonged operation of the spacecraft. Furthermore, solarcells 31 with a semiconductor contact structure that blocks or preventsmetal protrusion 15 would provide a more stable and improved total poweroutput over the life of a spacecraft due to a lower degradation rate.

There has, therefore, arisen a need to provide a metal migrationsemiconductor barrier layer that is able to suppress metal migrationfrom the metal contact of a semiconductor device into the semiconductoractive region under extreme environmental conditions, such as exposureto high temperatures for long times, as found, for example, forapplications in space, as well as on earth, under concentrated sunlight.There has further arisen a need to provide an appropriate material for ametal migration semiconductor barrier layer that is able to block themovement of metal within the semiconductor contact layer and to keep themetal away from the active region of the semiconductor device underextreme environmental conditions, such as found in space. There has alsoarisen a need to provide an improved solar cell for providing power to aspacecraft, such as a satellite, that will prolong the operation andimprove the performance of the spacecraft.

As can be seen, there is a need for a semiconductor structure with asemiconductor contact structure providing improved high temperaturereliability. There is a further need for providing a semiconductorstructure designed to keep any metal from entering the semiconductoractive region such that, consequently, the operation life of thesemiconductor structure will be extended. Also, there is a need forpreventing metal protrusion from the metal contact into thesemiconductor active layers during exposure of the semiconductorstructure to high temperatures for long times, improving the performanceand stability of the structure. Furthermore, there is a need forsuppression of metal migration into the semiconductor active layersunder extreme environmental conditions, such as exposure to hightemperatures for long times as found, for example, during spaceapplications. Moreover, there is a need for a method for forming asemiconductor structure for improving the high temperature reliabilityand the performance time of the semiconductor structure.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure that has one ormore metal migration semiconductor barrier layers incorporated within oroutside of a semiconductor contact layer and, therefore, provides acontact structure with improved high temperature reliability. Thepresent invention further provides a semiconductor structure that issuitable for, but not limited to, use in multifunction solar cells thatprovide power to spacecraft, for example, satellites. The presentinvention still further provides an appropriate semiconductor material,such as AlInP, that may be incorporated within the contact structure ofa semiconductor structure and that prevents metal protrusion from themetal contact into the semiconductor active layers during long-termexposure of the semiconductor structure to high temperatures, improvingthe performance, high temperature reliability, and stability of thesemiconductor structure. The present invention still further provides amethod for forming a semiconductor structure with metal migrationsemiconductor barrier layers and therefore, for improving the hightemperature reliability and the performance time at high temperatures ofthe semiconductor structure.

In one aspect of the present invention, a semiconductor structurecomprises a semiconductor substrate, a semiconductor active regionformed on the semiconductor substrate, a semiconductor contact layergrown on top of the semiconductor active region, a metal migrationsemiconductor barrier layer, and a metal contact deposited on thesemiconductor contact. The metal migration semiconductor barrier layeris embedded within the semiconductor contact layer.

In another aspect of the present invention, a semiconductor structurecomprises a semiconductor substrate, a semiconductor active regionformed on the semiconductor substrate, a metal migration semiconductorbarrier layer grown on top of the semiconductor active region, asemiconductor contact layer grown on top of the metal migrationsemiconductor barrier layer, and a metal contact deposited on thesemiconductor contact layer. The metal migration semiconductor barrierlayer is located entirely underneath the semiconductor contact layer,and the metal migration semiconductor barrier layer is in intimatecontact with the semiconductor contact layer.

In still another aspect of the present invention, a semiconductorstructure comprises a semiconductor substrate, a semiconductor activeregion formed on the semiconductor substrate, a semiconductor contactlayer grown on top of the semiconductor active region, a metal migrationsemiconductor barrier layer, and a metal contact deposited on the metalmigration semiconductor barrier layer. The metal migration semiconductorbarrier layer is grown on top of the semiconductor contact layer, suchthat the metal migration semiconductor barrier layer is located entirelyabove the semiconductor contact layer. The metal migration semiconductorbarrier layer is in intimate contact with the semiconductor contactlayer.

In a further aspect of the present invention, a solar cell devicestructure comprises a semiconductor substrate, a first set ofsemiconductor active layers being based on gallium arsenide grown on topof the semiconductor substrate, a second set of semiconductor activelayers being based on gallium indium phosphide grown on top of the firstset of semiconductor active layers, a semiconductor contact layer grownon top of the second set of semiconductor active layers, a metalmigration semiconductor barrier layer of aluminum indium phosphideembedded between the first region and the second region of thesemiconductor contact layer, and a metal contact. The semiconductorsubstrate has a thickness of about 100 to 300 microns. The first set ofsemiconductor active layers has a thickness of about 2 to 4 microns. Thesecond set of semiconductor active layers has a thickness of about 0.5to 1.0 microns. The semiconductor contact layer includes a first regionof gallium arsenide having a thickness of about 1000 to 5000 Å and adoping of about 1-5×10¹⁸ cm³¹ ³, a second region of gallium arsenidehaving a thickness of about 1000 to 5000 Å and a doping of 1-5×10¹⁸ cm³¹³, and a third region of gallium arsenide having a thickness ofapproximately 0 to 2000 Å and a doping of 1-5×10¹⁹ cm⁻³. The firstregion is grown on top of the second semiconductor active layer, thesecond region is grown on top of the first region, and the third regionis grown above the second region. The metal migration semiconductorbarrier layer is made of aluminum indium phosphide and has a thicknessin the range of 250 Å to 500 Å. The metal contact is atitanium/gold/silver metallization deposited on the semiconductorcontact layer. The metal contact includes a titanium layer having athickness of about 25 to 100 Å and is deposited on the third region ofthe semiconductor contact layer, a gold layer having a thickness ofabout 300 to 600 Å, wherein the gold layer is deposited on the titaniumlayer, and a silver layer having a thickness of about 40,000 to 60,000Å, wherein the silver layer is deposited on the gold layer.

In a still further aspect of the present invention, acell-interconnect-coverglass assembly comprises a solar cell, aplurality of interconnects welded onto the metal contact, and acoverglass covering the solar cell and the interconnects. The solar cellincludes semiconductor substrate, a semiconductor active region formedon the semiconductor substrate, a semiconductor contact layer grown ontop of the semiconductor active region, a metal migration semiconductorbarrier layer embedded within the semiconductor contact layer, and ametal contact deposited on top of the semiconductor contact layer.

In a still further aspect of the present invention, a contact structurecomprises a semiconductor contact layer, and a metal migrationsemiconductor barrier layer, wherein the metal migration semiconductorbarrier layer is in intimate contact with the semiconductor contactlayer.

In still another aspect of the present invention, a method for forming asemiconductor structure with metal migration semiconductor barrierlayers comprises the steps of: growing a semiconductor structure;blocking the metal migration from the metal contact towards thesemiconductor active region with the metal migration semiconductorbarrier layer; inhibiting metal/semiconductor reactions and formation ofundesirable phases within the semiconductor active region. Growing thesemiconductor structure includes the steps of: providing semiconductorsubstrate; forming a semiconductor active region on the semiconductorsubstrate; growing a semiconductor contact layer on top of thesemiconductor active region; embedding a metal migration semiconductorbarrier layer within the semiconductor contact layer; and depositing ametal contact on the semiconductor contact layer.

In still another aspect of the present invention, a method for forming asemiconductor structure with metal migration semiconductor barrierlayers comprises the steps of: growing a semiconductor structure;blocking the metal migration from the metal contact towards thesemiconductor active region with the metal migration semiconductorbarrier layer; inhibiting metal/semiconductor reactions and formation ofundesirable phases within the semiconductor active region. Growing thesemiconductor structure includes the steps of: providing semiconductorsubstrate; forming a semiconductor active region on the semiconductorsubstrate; growing a metal migration semiconductor barrier layer on topof the semiconductor active region; growing a semiconductor contactlayer on top of the metal migration semiconductor barrier layer; anddepositing a metal contact on the metal migration semiconductor barrierlayer.

In still another aspect of the present invention, a method for forming asemiconductor structure with metal migration semiconductor barrierlayers comprises the steps of: growing a semiconductor structure;blocking the metal migration from the metal contact towards thesemiconductor active region with the metal migration semiconductorbarrier layer; inhibiting metal/semiconductor reactions and formation ofundesirable phases within the semiconductor active region. Growing thesemiconductor structure includes the steps of: providing semiconductorsubstrate; forming a semiconductor active region on the semiconductorsubstrate; growing a semiconductor contact layer on top of thesemiconductor active region; growing a metal migration semiconductorbarrier layer on top of the semiconductor contact layer; and depositinga metal contact on the metal migration semiconductor barrier layer.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a typical prior artsemiconductor structure;

FIG. 2 is a schematic cross sectional view of a typical prior artsemiconductor structure with metal protrusion;

FIG. 3 is a perspective view of a cell-interconnect-coverglass assemblyof a typical prior art multifunction solar cell;

FIG. 4 is a schematic cross sectional view of a semiconductor structureaccording to one embodiment of the present invention;

FIG. 5 is a schematic cross sectional view of a semiconductor structurewith metal protrusion according to one embodiment of the presentinvention;

FIG. 6 is a schematic cross sectional view of a semiconductor structureaccording to another embodiment of the present invention;

FIG. 7 is a schematic cross sectional view of a semiconductor structureaccording to another embodiment of the present invention; and

FIG. 8 is a schematic cross sectional view of a solar cell devicestructure according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Broadly, an embodiment of the present invention provides a semiconductorstructure with a contact structure for improved high temperaturereliability. Contrary to the known prior art, the contact structure mayinclude one or more metal migration semiconductor barrier layersembedded within or in intimate contact with a semiconductor contactlayer that may be designed to block the movement of metal from the metalcontact towards the semiconductor active region. The semiconductorstructure as in one embodiment of the present invention may be used, forexample, for III-V based multifunction solar cells that provide power tospacecraft, such as satellites. The semiconductor structure as in oneembodiment of the present invention may further be used, for example,for III-V based terrestrial solar cells that typically operate underconcentrated sunlight. Furthermore, an embodiment of the presentinvention provides a method for forming a semiconductor structure withmetal migration semiconductor barrier layers for improving the hightemperature reliability and the lifetime of the semiconductor structurecompared to known prior art semiconductor structures.

In one embodiment, the present invention provides a semiconductorstructure that has one or more metal migration semiconductor barrierlayers incorporated within or outside of, but in intimate contact with,a semiconductor contact layer that prevent metal protrusion from themetal contact into the active region of the semiconductor structureduring exposure of the semiconductor structure to high temperatures forlong periods of time. By incorporating such metal migrationsemiconductor barrier layers into the contact structure of thesemiconductor structure the performance, stability, and reliability ofthe semiconductor structure at high temperatures may be improved incomparison to prior art semiconductor structures that use a metalbarrier layer.

An embodiment of the present invention further provides semiconductormaterials, such as AIInP, that can be incorporated into the contactstructure of a semiconductor device, for example, a multijunction III-Vbased solar cell, to prevent metal migration from the metal contact intothe semiconductor active layers under extreme environmental conditions,such as exposure to high temperatures for extended amounts of time. Theuse of semiconductor materials, such as AIInP, as metal migrationbarrier materials instead of prior art metals with high temperaturestability, such as Pt and Pd, enables the prevention of metal protrusionfrom the metal contact into the active region of the semiconductorstructure even during long-term exposure to high temperatures, such asfound in space applications. Therefore, contrary to prior artsemiconductor structures, the semiconductor structures according to oneembodiment of the present invention will exhibit enhanced performanceduring long-term exposure to high temperatures.

An embodiment of the present invention further provides at least onemetal migration semiconductor barrier layer that is incorporated withinthe semiconductor contact layer of a semiconductor structure. Incontrast of the known prior art, the metal migration semiconductorbarrier layer as in one embodiment of the present invention is designedto block any movement of metal that might occur within the semiconductorcontact layer and, therefore, to prevent any metal from entering thesemiconductor active layers, which would cause the failure of thesemiconductor structure. Contrary to prior art metal barrier layers, themetal migration semiconductor barrier layer according to one embodimentof the present invention is proven to work under long-term hightemperature conditions, as found in space applications. By incorporatingmetal migration semiconductor barrier layers, as in one embodiment ofthe present invention, into the contact structure of a semiconductorstructure, a contact structure with improved high temperaturereliability, as needed, for example, in space applications may bedesigned.

By providing a contact structure according to one embodiment of thepresent invention, the high temperature reliability of a semiconductordevice, such as a III-V based multifunction solar cell, can be improvedin contrast to prior art semiconductor devices. Furthermore, theoperation life of the semiconductor device under long-term hightemperature conditions will be prolonged, which provides an advantageover prior art.

Referring now to FIG. 4, a schematic cross sectional view of asemiconductor structure 40 is illustrated according to one embodiment ofthe present invention. The semiconductor structure 40 includes asemiconductor substrate 41, a semiconductor active region 42, asemiconductor contact layer 43, a metal contact 44, and at least onemetal migration semiconductor barrier layer 45. The semiconductor activeregion 42 may include multiple semiconductor active layers such as 81and 82 (as shown in FIG. 8). The semiconductor active layers may bedeposited on the semiconductor substrate 41. The semiconductor contactlayer 43 may then be grown on top of the semiconductor active region 42.The metal contact 44 may then be deposited on the semiconductor contactlayer 43 in a separate process.

The semiconductor contact layer 43 is generally considered to be aninactive part of the semiconductor structure 40, since the inherentperformance of the semiconductor structure 40 is not affected by thesemiconductor contact layer 43. However, the semiconductor contact layer43 is an important part of the semiconductor structure 40, since it isdesigned to be in intimate physical contact with the metal contact 44and must provide sufficiently low contact resistance in order tominimize the impact on the overall semiconductor structure 40performance. The present invention involves modifying the semiconductorcontact layer 43. The semiconductor contact layer 43 and at least onemetal migration semiconductor barrier layer 45 may form a contactstructure 47. As shown in FIG. 4, at least one metal migrationsemiconductor barrier layer 45 may be embedded within the semiconductorcontact layer 43 according to one embodiment of the present invention.FIG. 4 illustrates the metal migration semiconductor barrier layer 45incorporated approximately in the middle of the semiconductor contactlayer 44. However, the metal migration semiconductor barrier layer 45may be placed anywhere within the semiconductor contact layer 43.Furthermore, more than one metal migration semiconductor barrier layer45 may be embedded within the semiconductor contact layer 43. Thelocation of the metal migration semiconductor barrier layer 45 withinthe semiconductor contact layer 43 as well as the number of metalmigration semiconductor barrier layers 45 may influence theeffectiveness of the contact structure 47.

Referring now to FIG. 5, a schematic cross sectional view of asemiconductor structure 40 with metal protrusion 46 is illustratedaccording to one embodiment of the present invention. The semiconductorstructure 40 has been exposed to high temperatures for a long time,similar to conditions found during applications in space. The prolongedexposure to high temperatures may cause a portion of the metal contact44 to diffuse or migrate downward towards the semiconductor activeregion 42. Any metal reaching the active region 42 comprises a highlyundesirable situation, as the metal atoms will likely short out thesemiconductor P/N junction and may cause the structure 40 to fail. Themetal migration semiconductor barrier layer 45 may function as a barrierto stop metal migration from the metal contact 44 and may prevent themetal protrusion 46 to extend into the semiconductor active region 42,as shown in FIG. 5.

Referring now to FIG. 6, a schematic cross sectional view of asemiconductor structure 60 is illustrated according to anotherembodiment of the present invention. The semiconductor structure 60 mayinclude a semiconductor substrate 41, a semiconductor active region 42,a semiconductor contact layer 43, a metal contact 44, and a metalmigration semiconductor barrier layer 45. The metal migrationsemiconductor barrier layer 45 may be grown on top of the semiconductoractive region 42 followed by the semiconductor contact layer 43, suchthat the metal migration semiconductor barrier layer 45 may be locatedentirely underneath and in intimate contact with the semiconductorcontact layer 43. The semiconductor contact layer 43 and the metalmigration semiconductor barrier layer 45 may form a contact structure47.

Referring now to FIG. 7, a schematic cross sectional view of asemiconductor structure 70 is illustrated according to anotherembodiment of the present invention. The semiconductor structure 70 mayinclude a semiconductor substrate 41, a semiconductor active region 42,a semiconductor contact layer 43, a metal contact 44, and a metalmigration semiconductor barrier layer 45. The semiconductor contactlayer 43 may be grown on top of the semiconductor active region 42. Themetal migration semiconductor barrier layer 45 may be grown on top ofthe semiconductor contact layer 43, such that the metal migrationsemiconductor barrier layer 45 may be located entirely above and inintimate contact with the semiconductor contact layer 43. Thesemiconductor contact layer 43 and the metal migration semiconductorbarrier layer 45 may form a contact structure 47.

Independent from the location of the metal migration semiconductorbarrier layer 45 proximate to the semiconductor contact layer 43 andindependent from the number of metal migration semiconductor barrierlayers 45 embedded within the semiconductor compact layer 43, thecombination of the semiconductor contact layer 43 and the metalmigration semiconductor barrier layer 45 may provide a contact structure47 that may improve the reliability of a semiconductor structure 40, 60,or 70 during exposure to high temperatures for prolonged time periods.

The semiconductor structure 40 (as illustrated in FIGS. 4 and 5), thesemiconductor structure 60 (as illustrated in FIG. 6), and thesemiconductor structure 70 (as illustrated in FIG. 7), all may be grownusing a growth method known as metal organic vapor phase epitaxy(MOVPE). The use of other growth methods may also be possible. Theeffectiveness of the metal migration semiconductor barrier layer 45 hasbeen demonstrated on III-V based multijunction solar cells. The use ofthe metal migration semiconductor barrier layer 45 as in FIGS. 4, 5, 6,and 7 is not limited to III-V based multijunction solar cells. The metalmigration semiconductor barrier layer 45 may be incorporated into thecontact structure of any semiconductor structure that is used forapplications under prolonged high temperature conditions.

The preferred material for the metal migration semiconductor barrierlayer 45 (as shown in FIGS. 4, 5, 6, and 7) may be aluminum indiumphosphide, but the material for the metal migration semiconductorbarrier layer may be extended to other phosphide compounds, such asgallium indium phosphide, and aluminum containing compounds, such asaluminum gallium arsenide.

Referring now to FIG. 8, a schematic cross sectional view of a solarcell device structure 80 is illustrated according to another embodimentof the present invention. The solar cell device structure 80 may includea semiconductor substrate 41, a semiconductor active region 42, asemiconductor contact layer 43, a metal contact 44, and a metalmigration semiconductor barrier layer 45. The metal migrationsemiconductor barrier layer 45 may be embedded within the semiconductorcontact layer 43 forming a contact structure 47. The semiconductorsubstrate 41 may be an elemental semiconductor such as silicon orgermanium, or a III-V compound semiconductor such as gallium arsenide orindium phosphide. The semiconductor substrate 41 may have a thickness ofabout 100-300 microns. The semiconductor active region 42 may include afirst set of semiconductor active layers 81 being based on galliumarsenide and a second set of semiconductor active layers 82 being basedon gallium indium phosphide. The first set of semiconductor activelayers 81 may have a thickness of about 2-4 microns and may be grown ontop of the semiconductor substrate 41. The second set of semiconductoractive layers 82 may have a thickness of about 0.5-1.0 microns and maybe grown on top of the first set of semiconductor active layers 81. Thesemiconductor contact layer 43 grown on top of the second set ofsemiconductor active layers 82 may include a first region 83 of galliumarsenide having a thickness of approximately 1000-5000 Å and a doping of1-5×10¹⁸ cm³¹ ³, a second region 84 of gallium arsenide having athickness of approximately 1000-5000 Å and a doping of 1-5×10¹⁸ cm⁻³,and a third region 85 of gallium arsenide having a thickness ofapproximately 0-2000 Å and a doping of 1-5×10¹⁹ cm⁻³. The semiconductorcontact layer 43 may also be grown from gallium indium arsenide with1-3% indium. The metal migration semiconductor barrier layer 45 ofaluminum indium phosphide with a thickness in the range of 250 to 500 Åmay be embedded between the first region 83 and the second region 84 ofthe semiconductor contact layer 43. Other materials, such as aluminumgallium arsenide, gallium indium phosphide, gallium indium arsenide withhigher indium contents, strain-balanced superlattices, strained layers,or other lattice-matched materials may be used as materials for themetal migration semiconductor barrier layer 45. The metal contact 44 maybe a conventional titanium/gold/silver metallization and may include a25-100 Å thick titanium layer 86, followed by a 300-600 Å thick goldlayer 87, and a 40,000-60,000 Å thick silver layer 88. Other materials,for example, titanium/platinum/silver or modifications of theconventional titanium/gold/silver metallization may be used as materialfor the metal contact 44.

The semiconductor contact layer 43 may have the metal migrationsemiconductor barrier layer 45 embedded in it, as shown in FIG. 8, suchthat the metal migration semiconductor barrier layer 45 acts as abarrier against the diffusion of metal species, such as gold (87) orsilver (88), from the metal contact 44 towards the active region 42. Themetal migration semiconductor barrier layer 45 may further inhibitmetal/semiconductor reactions and the formation of undesirable phasesthat can take place during a long-term exposure to high temperatures.

A method for forming a semiconductor structure 40 with metal migrationsemiconductor barrier layers 45 and therefore, for improving the hightemperature reliability and the performance time at high temperatures ofthe semiconductor may include the steps of: growing a semiconductorstructure 40 by depositing a semiconductor active region 42 on asemiconductor substrate 41; growing a semiconductor contact layer 43 ontop of the semiconductor active region 42; embedding a metal migrationsemiconductor barrier layer 45 within the semiconductor contact layer43; depositing a metal contact 44 on top of the semiconductor contactlayer 43; blocking the metal migration from the metal contact 44 towardsthe semiconductor active region 42; inhibiting metal/semiconductorreactions and formation of undesirable phases; improving the hightemperature reliability of the semiconductor structure 40; and extendingthe operation life of the semiconductor structure 40. More than onemetal migration semiconductor barrier layers 45 may be embedded into thesemiconductor contact layer 43. Furthermore, instead of being embeddedinto the semiconductor contact layer 43, the metal migrationsemiconductor barrier layer 45 may be deposited underneath and inintimate contact with the semiconductor contact layer 43 or above and inintimate contact with the semiconductor contact layer 43.

Therefore, by providing a contact structure 47 that includes thesemiconductor contact layer 43 and the metal migration semiconductorbarrier layer 45 the reliability of a semiconductor structure 40, 60,70, or 80 during long-term exposure to high temperatures may be improvedand the operation time of the semiconductor structure 40, 60, 70, or 80may be extended. Moreover, by using a solar cell 80 as in one embodimentof the present invention, for example, in a cell-interconnect-coverglassassembly 30 (as shown in FIG. 3), the high temperature reliability ofthis assembly 30 may be improved and the operation time may beprolonged, extending the life of the satellite that the solar cell 80 ispowering. Although the contact structure 47 of the present invention maybe used most effectively in III-V based multifunction solar cells thatprovide power to spacecraft, for example, satellites, otherapplications, for example, terrestrial solar cells, are possible.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A semiconductor structure, comprising: a semiconductor substrate; asemiconductor active region formed on said semiconductor substrate; asemiconductor contact layer grown on top of said semiconductor activeregion; a metal migration semiconductor barrier layer; wherein saidmetal migration semiconductor barrier layer is embedded within saidsemiconductor contact layer; and a metal contact deposited on saidsemiconductor contact layer.
 2. The semiconductor structure of claim 1,further comprising at least one additional metal migration semiconductorbarrier layer, wherein said at least one additional metal migrationsemiconductor barrier layer is embedded within said semiconductorcontact layer.
 3. The semiconductor structure of claim 1, wherein saidmetal migration semiconductor barrier layer is embedded within saidsemiconductor contact layer proximate to the center of saidsemiconductor contact layer.
 4. The semiconductor structure of claim 1,wherein said metal migration semiconductor barrier layer is made ofaluminum indium phosphide.
 5. A semiconductor structure, comprising: asemiconductor substrate; a semiconductor active region formed on saidsemiconductor substrate; a metal migration semiconductor barrier layergrown on top of said semiconductor active region; a semiconductorcontact layer grown on top of said metal migration semiconductor barrierlayer, such that said metal migration semiconductor barrier layer islocated entirely underneath said semiconductor contact layer, andwherein said metal migration semiconductor barrier layer is in intimatecontact with said semiconductor contact layer; and a metal contactdeposited on said semiconductor contact layer.
 6. The semiconductorstructure of claim 5, wherein the material of said metal migrationsemiconductor barrier layer is a phosphide compound.
 7. Thesemiconductor structure of claim 5, wherein said semiconductor activeregion includes a first set of semiconductor active layers being basedon gallium arsenide and a second set of semiconductor active layersbeing based on gallium indium phosphide.
 8. The semiconductor structureof claim 7, wherein said first set of semiconductor active layers isgrown on top of said semiconductor substrate and wherein said second setof semiconductor active layers is grown on top of said first set ofsemiconductor active layers.
 9. A semiconductor structure, comprising: asemiconductor substrate; a semiconductor active region formed on saidsemiconductor substrate; a semiconductor contact layer grown on top ofsaid semiconductor active region; a metal migration semiconductorbarrier layer; wherein said metal migration semiconductor barrier layeris grown on top of said semiconductor contact layer, such that saidmetal migration semiconductor barrier layer is located entirely abovesaid semiconductor contact layer, and wherein said metal migrationsemiconductor barrier layer is in intimate contact with saidsemiconductor contact layer; and a metal contact deposited on said metalmigration semiconductor barrier layer.
 10. The semiconductor structureof claim 9, wherein the material of said metal migration semiconductorbarrier layer is an aluminum-containing compound.
 11. The semiconductorstructure of claim 9, wherein said metal contact is atitanium/gold/silver metallization.
 12. The semiconductor structure ofclaim 9, wherein the material of said semiconductor contact layer isgallium arsenide.
 13. A solar cell device structure, comprising: asemiconductor substrate, wherein said semiconductor substrate has athickness of about 100 to 300 microns; a first set of semiconductoractive layers being based on gallium arsenide, wherein said first set ofsemiconductor active layers has a thickness of about 2 to 4 microns, andwherein said first set of semiconductor active layers is grown on top ofsaid semiconductor substrate; a second set of semiconductor activelayers being based on gallium indium phosphide, wherein said second setof semiconductor active layers has a thickness of about 0.5 to 1.0microns, and wherein said second set of semiconductor active layers isgrown on top of said first set of semiconductor active layers; asemiconductor contact layer grown on top of said second set ofsemiconductor active layers, including: a first region of galliumarsenide having a thickness of about 1000 to 5000 Å and a doping of1-5×10¹⁸ cm⁻³; a second region of gallium arsenide having a thickness ofabout 1000 to 5000 Å and a doping of 1-5×10¹⁸ cm⁻³; and a third regionof gallium arsenide having a thickness of approximately 0 to 2000 Å anda doping of 1-5×10¹⁹ cm⁻³; wherein said first region is grown on top ofsaid second semiconductor active layer, wherein said second region isgrown on top of said first region, and wherein said third region isgrown above said second region; a metal migration semiconductor barrierlayer of aluminum indium phosphide having a thickness in the range of250 Å to 500 Å, and wherein said metal migration semiconductor barrierlayer is embedded between said first region and said second region ofsaid semiconductor contact layer; a metal contact, wherein said metalcontact is a titanium/gold/silver metallization deposited on saidsemiconductor contact layer, and wherein said metal contact includes: atitanium layer having a thickness of about 25 to 100 Å, wherein saidtitan layer is deposited on said third region of said semiconductorcontact layer; a gold layer having a thickness of about 300 to 600 Å,wherein said gold layer is deposited on said titan layer; and a silverlayer having a thickness of about 40,000 to 60,000 Å, wherein saidsilver layer is deposited on said gold layer.
 14. The solar cell devicestructure of claim 13, wherein said semiconductor substrate is anelemental semiconductor.
 15. The solar cell device structure of claim13, wherein said semiconductor substrate is a III-V compoundsemiconductor.
 16. The solar cell device structure of claim 13, whereinthe material of said metal migration semiconductor barrier layer isselected from the group consisting of aluminum gallium arsenide, galliumindium phosphide, gallium indium arsenide, strain-balancedsuperlattices, strained layers, and lattice-matched materials.
 17. Thesolar cell device structure of claim 13, wherein said metal contact is atitanium/platinum/silver metallization.
 18. Acell-interconnect-coverglass assembly, comprising: a solar cell,including: a semiconductor substrate; a semiconductor active regionformed on said semiconductor substrate; a semiconductor contact layergrown on top of said semiconductor active region; a metal migrationsemiconductor barrier layer embedded within said semiconductor contactlayer; and a metal contact deposited on top of said semiconductorcontact layer; a plurality of interconnects welded onto said metalcontact; and a coverglass covering said solar cell and saidinterconnects.
 19. The cell-interconnect-coverglass assembly of claim18, wherein said semiconductor substrate is selected from the groupconsisting of elemental semiconductors and III-V compoundsemiconductors.
 20. The cell-interconnect-coverglass assembly of claim18, wherein said solar cell provides power to a spacecraft.
 21. Thecell-interconnect-coverglass assembly of claim 18, wherein said solarcell provides power to a satellite.
 22. A contact structure, comprising:a semiconductor contact layer; and a metal migration semiconductorbarrier layer, wherein said metal migration semiconductor barrier layeris in intimate contact with said semiconductor contact layer.
 23. Thecontact structure of claim 22, wherein said metal migrationsemiconductor barrier layer is embedded within said semiconductorcontact layer.
 24. The contact structure of claim 23, further comprisingan additional metal migration semiconductor barrier layer embeddedwithin said semiconductor contact layer.
 25. The contact structure ofclaim 22, wherein said semiconductor contact layer is grown on top ofsaid metal migration semiconductor barrier layer, such that said metalmigration semiconductor barrier layer is located entirely underneathsaid semiconductor contact layer.
 26. The contact structure of claim 22,wherein said metal migration semiconductor barrier layer is grown on topof said semiconductor contact layer, such that said metal migrationsemiconductor barrier layer is located entirely above said semiconductorcontact layer.
 27. The contact structure of claim 22, wherein saidsemiconductor contact layer and said metal migration semiconductorbarrier layer are part of a semiconductor device.
 28. The contactstructure of claim 22, wherein said semiconductor contact layer and saidmetal migration semiconductor barrier layer are part of a III-V basedmultifunction solar cell.
 29. The contact structure of claim 22, whereinthe material of said metal migration semiconductor barrier layer isaluminum indium phosphide.
 30. A method for forming a semiconductorstructure with metal migration semiconductor barrier layers, comprisingthe steps of: growing a semiconductor structure, including the steps of:providing semiconductor substrate; forming a semiconductor active regionon said semiconductor substrate; growing a semiconductor contact layeron top of said semiconductor active region; embedding a metal migrationsemiconductor barrier layer within said semiconductor contact layer; anddepositing a metal contact on said semiconductor contact layer; blockingthe metal migration from said metal contact towards said semiconductoractive region with said metal migration semiconductor barrier layer; andinhibiting metal/semiconductor reactions and formation of undesirablephases within said semiconductor active region.
 31. The method of claim30, further including the step of: embedding one additional metalmigration semiconductor barrier layer within said semiconductor contactlayer.
 32. A method for preventing metal migration from a contact layerinto an active layer of a semiconductor structure, comprising the stepsof: growing a semiconductor structure, including the steps of: providingsemiconductor substrate; forming a semiconductor active region on saidsemiconductor substrate; growing a metal migration semiconductor barrierlayer on top of said semiconductor active region; growing asemiconductor contact layer on top of said metal migration semiconductorbarrier layer; and depositing a metal contact on said metal migrationsemiconductor barrier layer; blocking the metal migration from saidmetal contact towards said semiconductor active region with said metalmigration semiconductor barrier layer; and inhibitingmetal/semiconductor reactions and formation of undesirable phases withinsaid semiconductor active region.
 33. A method for preventing metalmigration from a contact layer into an active layer of a semiconductorstructure, comprising the steps of: growing a semiconductor structure,including the steps of: providing semiconductor substrate; forming asemiconductor active region on said semiconductor substrate; growing asemiconductor contact layer on top of said semiconductor active region;growing a metal migration semiconductor barrier layer on top of saidsemiconductor contact layer; and depositing a metal contact on saidmetal migration semiconductor barrier layer; blocking the metalmigration from said metal contact towards said semiconductor activeregion with said metal migration semiconductor barrier layer; andinhibiting metal/semiconductor reactions and formation of undesirablephases within said semiconductor active region.